Rapid scanning double focusing mass spectrograph

ABSTRACT

A rapid scanning double focusing mass spectrograph having a source of ions, means for producing a toroidal electric field, means for producing a homogeneous magnetic field and means for producing an r 1 magnetic field, wherein the r 1 magnetic field and the homogeneous magnetic field are excited simultaneously with a single energizing magnet and upper and lower parallel pole pieces of the means for producing a homogeneous magnetic field are secured to the rear ends of the upper and lower conical pole pieces of the means for producing an r 1 magnetic field with the curvature radii of the two means being kept equal.

1 1 Feb. 6, 1973 1 1 RAPID SCANNING DOUBLE FOCUSING MASS SPECTROGRAPII [75] Inventor:

[73] Assignee: Nihon Denski Kabushiki Kaisha,

Tokyo, Japan June 15, 1970 I-Iisashi Matsuda, Toyonaka, Japan [22] Filed:

21 Appl. N0.: 46,178

Related [1.8. Application Data [63] Continuation-in-part 0f Ser. No. 702,469, Feb. 1,

Washburn ..250/41 .9

OTHER-PUBLICATIONS A New Mass Spectrograph With Very Large Dispersion" by H. Matsuda et al. from Zeitschrift Fur Naturforschung," Vol. 21a, 1966, pp. 25-33.

. Primary Examiner-William F. Lindquist Attorney-Webb, Burden, Robinson & Webb [57] ABSTRACT A rapid scanning double focusing mass spectrograph having a source of ions, means for producing a toroidal electric field, means for producing a homogeneous magnetic field and means for producing an r magnetic field, wherein the r" magnetic field and the homogeneous magnetic field are excited simultaneously with a single energizing magnet and upper and lower parallel pole pieces of the means for producing a homogeneous magnetic field are secured to the rear ends of the upper and lower conical pole pieces of the means for producing an r"' magnetic field with the curvature radii of the two means being kept equal.

6 Claims, 3 Drawing Figures RAPID SCANNING DOUBLE FOCUSING MASS SPECTROGRAPH This application. is a continuation-in-part of application Ser. No. 702,469, filed Feb. 1, 1968, now abandoned.

My invention relates to rapid scanning mass spectrographs or spectrometers having large mass dispersion and high resolving power for the precise determination of atomic masses. In particular, my invention relates to double focusing mass spectrographs.

The requirements of the magnetic field or fields in a mass spectrometer are twofold: they must separate the ions in an ion beam according to their mass to charge (m/e) ratios. At the same time they must focus the paths of ions having the same m/e ratios but which enter the field along slightly different paths. in the simple mass spectrometer, a uniform magnetic field causes the greater dispersion of ions having different m/e ratios after the ions have been turned through 180. This is also the point of maximum focusing for the paths of ions having the same m/e ratio. Beyond 180 the focus is lessened and the dispersion decreases. Hypothetically, at 360 the paths of all ions regardless of mass to charge ratio converge on the point at which they entered the field.

In a simple mass spectrometer that focuses the beam at the end of a 180 turn, the only manner in which dispersion can be increased, is to increase the radius of curvature of the ion path. To increase the radius of curvature of the ion path, it is necessary to use larger and larger pole pieces and cores-which become awkward and unwieldy for the large dispersions desired for analysis of organic compounds. 7

Also, in a double focusing mass spectrograph, by enlarging the radius of the ion orbit, it is possible to obtain high resolutions. For technical reasons, it is very difficult to sufficiently enlarge the linear dimensions of the apparatus thereby making this method rather impractical.

Recently, a new type of mass spectrograph with a large mass dispersion that provides an ultra high resolution of several hundred thousands or more without requiring large linear dimensions has been developed.

This has been accomplished by utilization of an r" magnetic field for the mass dispersing action. The r' magnetic field, which I first proposed, is characterized by a magnetic field strength in the Y-direction, i.e., in the radial direction, that decreases in accordance with the increase in the radius of the magnetic field, according to the formula:

wherein B the magnetic field strength along the central orbit,

ln a mass analyzing device of that kind, a sector type toroidal electric field having a deflection angle of l 18.7 and a means radius'of 30cm is used as the first focusing field to compensate for the large energy dispersion that is caused by the r magnetic field. For

the second focusing field, a homogeneous magnetic field of cm mean radius is used to focus ions having the same mass on a photographic plate or the like. A sector type r' magnetic field having a deflecting angle of l98.l and a mean radius of 22cm, the strength of which varies in proportion to r, is inserted between the toroidal electric field and the homogeneous magnetic field.

With this arrangement consisting of three fields, two focusing and one dispersing, the double focusing condition is given by (Z/Ke) sin Ke d) e= sin hi (5m where e is the electric field characterizing constant, and (be, m and m3 are the deflecting angles of the toroidal electric field, dispersing magnetic field and focusing magnetic field, respectively.

In order to reproduce an extremely high resolution mass spectrum, (M/A M; 500,000 or better), it is necessary to precisely adjust the electric currents of the exciting coils producing the two magnetic fields so as to prevent the ion beam from deviating from the original optimum path of travel. in instruments designed according to the prior art, the ion beam, after passing through the toroidal electric field, deviates from the optimum path of travel when dispersing magnetic field intensity is changed, since the dispersing magnet and focusing magnet coils are energized by different magnet power supplies. Extremely precise control of the focusing magnetic field intensity is necessary to restore the beam to its optimum path of travel in accordance with the change in the dispersing magnetic field intensity. Even when exercising the aforementioned care during control, it is very difficult to adjust the strength of the dispersing magnetic field so as to maintain the intensity ratio between the two magnetic fields constant at all times, because of the presence of residual electromagnetic hysteresis. Furthermore, the strengths of the two magnetic fields are required to be varied in proportion to each other and at the same time the intensity ratio between the magnetic fields must be maintained constant at all times with a stability of an order of more than 10'. It is impossible, however, to detect the intensity ratio between the two magnetic fields with an accuracy of more than 10' when using a Hall element.

As a result, the optimum operating condition of the mass spectrograph cannot be attained without precise and time consuming adjustments which includes the actual emittance of ions from the ion source to the detector for travel along the optimum ion orbit that originally provided the high resolution mass spectrum. Consequently, each time the power supplied to energize the dispersing magnet and focusing magnet is changed in accordance with the change in the measuring condition, it is necessary to repeat the entire procedure several times until the optimum operating condition of the two magnetic fields, both with respect to beam position and beam focusing, is attained.

'Furthermore, due to the presence of the residual electromagnetic hysteresis, it is extremely difficult to scanthe mass spectrum ofa sample which requires that the intensity ratio between the two magnetic fields be exciting current to the respective magnetsmust be adequately stabilized it is necessary to provide magnetic field exciting current stabilizers, which arenot only very expensive, but tend to make the entire ap paratus very large and bulky.

My invention provides an improved mass spectrograph that overcomes the aforementioned disadvantages, so that a resolving power in the order of from 100,000-300,000 over the entire range is ensured by making adjustment at only one point of the mass to charge ratio of the charged particles.

In practice, the intensities of the r' magnetic field and homogenious magnetic field cannot be scanned at high speed simultaneously and yet, with this invention, it is possible. Moreover, since the above described high resolving power can be obtained over a very wide range with this high speed scanning, the mass to charge ratio of each spectrum can be measured accurately.

Other novel features and advantages will become apparent by reading the following detailed specification in conjunction with the accompanying drawings in which:

FIG. 1 is a plan view, partly broken away, of the focusing and dispersing means of my invention;

FIG. 2 is a cross-section of the invention taken on line A-ZB of FIG. 1; and, i

FIG. 3 is a cross-section of my invention taken on line C-Z-D of FIG. 1.

. maintained constant at all times. Moreover, since the ing effect on the ion beam in the axial direction, that is, I

in the direction vertical to the plane determined by the orbit. In order to obtain optimum mass dispersion, it is preferable to make the deflecting angle 4) m as large as possible. For example, taking into consideration the focusing effect, an angle of 200.32 could be used where am =cm. Though not shown in FIG. 1, a toroidal electric field is arranged as the first focusing field. The deflection angle of the toroidal electric field having 30cm curvature radius is l 12.15". A homogeneous magnetic field 4 serves as the second focusing field. The curvature radius am of theion beam, in this case, is also 30cm to permit the focusing magnetic'field to be placed immediately after the dispersing magnetic field. The deflecting angle 4: m of the focusing magnetic field is 13.59 which is determined by the double focusing condition. From the double focusing condition it is apparent that curvature radii of the ion beam in the three fields, the distance between the toroidal electric field and the dispersing magnetic field, and the distance between the dispersing magnetic field and the focusing magnetic field are independent of the double focusing condition. Herzog shunts Sand 6 (an explanation of the Briefly according to applicants invention, a r? magnetic field and a homogeneousmag'netic field are placed about the same reasonably sized magnetic core. The two magnetic fields having the same curvature radius are energized with a single magnet. The magnetic core is energized through a scanner and an excitation source. Dispersion is not determined by the radius of curvature of the ion path. It' is controlled by the angle through which the ion beam is turned. The r' field does not focus the ion beams at the end of a 180 turn. Ions of different m/e ratios-emerge from the r"' magnetic field at different angles; however, all ions mechanisms has been omitted for the sake of clarity) are arranged at entrance 7 of the dispersing field and at exit 8 of the focusing magnetic field to reduce stray magnetic fields as much as possible and for the realization of a good focusing condition. Beam current monitors 9, 10 and 11 (an explanation of the mechanisms has also been omitted for the sake of clarity) are arranged at points along the ion path for monitoring purposes. Knock pins 12a12b, 12c and 12d made of nonmagnetic material are arranged in holes in the upper cylindrical supports 13a, 13b, 13c and 13d so as to be in sliding contact with the upper cylindrical supports. R0-

having the same m/e are traveling in parallel paths. The

homogeneous field following. the r magnetic field focuses the ions emerging from the r field. The focal over a range of intensities such that the intensity ratio between both fields is maintained constant. In this way, a high resolving power over the entire scanning range is achieved.

Referring to FIG. 1, and rmagnetic field 1 and a central ion beam path 2 having a curvature radius am for example 30cm, are dependent upon the size of energizing magnet 3. The rmagnetic field has a focus 7 tary knobs 14a, 14b, 14c and 14d are mounted on the exterior of an upper conical pole piece 24a and a lower conical pole piece (not shown) so as to impart forward and backward movement to four distant wedge-shaped pieces of phosphor bronze 31, one of which is shown in FIG. 3, arranged between the respective upper collars 16a, 16b, 16c and 16d and lower collars 17a, l7b, l7c

and 17d. The upperyoke plate 18 is secured to the upper surface of the centrally arranged core 19 of magnetic material by means of three screws 20a, 20b and 20c. Though not shown, the lower yoke plate is also securedto the lower surface of the core .19 in a like manner. By means of this arrangement, the upper and lower yoke plates are electromagnetically connected to the core, on which an exciting coil 21 is accommodated. Z indicates the vertical axis as the center of the two magnetic fields. An analyzer tube 22 made of nonmagnetic material is housed in a gap formed in the peripheries of the conical and parallel pole pieces. The interior of the tube is maintained at a high vacuum by means of a vacuum pump (not shown), connected to the tube via a conduit 23.

To enable the r' magnetic field to be generated exactly in the median plane, that is, in the plane determined by the central orbit, the upper and lower conical pole pieces 24a and 24b, made of soft iron, are symmetrically arranged with respect to the plane of symmetry F as shown in FIG. 2.

In addition a 15mm gap G at the mean radius is provided between the confronting conical pole faces 25a and 25b and the conical pole faces are shaped so as to converge at point C that coincides with the intersection point 0 of the plane F of symmetry and the axis Z which is vertical to the plane. In other words, the confronting conical pole faces 25a and 25b are disposed so that the distance r between the path of the central ion beam and the intersection point C of the converging extension lines 13-13 is equal to the curvature radius a of the ion beam in the dispersing magnetic field.

However, the nonuniformity of gap G between the confronting conical pole pieces and variations in the magnetic characteristics of the magnetic conical pole pieces introduces asymmetries in the r" magnetic field that have a disturbing effect on the path of the ion beam entering or leaving the dispersing magnetic field. Though great care is usually employed in selecting and machining the pole piece assembly, additional correction is necessary because of the nonuniformity and variations to ensure that the dispersing magnetic field is distributed exactly in proportion to r' at all points around the ion beam path, thus ensuring a high resolution mass spectrum.

. The gapbetween the parallel pole faces of the focusing magnetic field is not mechanically controllable and, therefore, the strength of the focusing magnetic field necessarily has to be controlled by varying the exciting current of the energizing magnet to maintain the ion romagnetic material, abuts the upper and lower yoke platesl8 and 26. On the lower yoke plate the lower conical pole piece 24b is rigidly mounted. The upper and lower parallel pole pieces 27a and 27b with confronting upper and lower parallel pole faces 36a and 36b are intercrossing with the confronting upper and lower conical pole faces a and 25b in points H and H which are located directly above and below path 2 of the central ion beam on the rear ends P and P of the upper and lower conical pole pieces 24a and 24b, so that the curvature radius of the ion beam in the focusing magnetic field can be maintained equal to that of the ion beam in the dispersing magnetic field.

In the operation of the mass spectrograph, according to the present invention, it is also necessary to adjust pole gap G in the first place so that the r' magnetic field is distributed exactly in proportion to rat all points around the ion beam path. Towards this end, an air gap K, for example, 0.5mm, is provided between the lower surface of the upper yoke plate and the upper surfaces of the upper conical and parallel pole pieces.

. Knock pin 12a is slidably arranged in holes 29a and 30a conical pole pieces 24a and 24b by means of screws 32. Control knob 14a is connected to a shaft 33 in torque transfer relation, the inner end of the shaft is provided with a screw thread 34 which is rotatable within the distant wedge-shaped piece 31. The screw thread, when rotated by means of the control knob 14a, imparts an upward and downward movement to upper collar 16a and hence to the upper conical pole piece 24a. At the same time other wedge-shaped pieces are also controlled in a like manner by the respective control knobs 14b, 14c and 14d, thus moving the upper conical pole piece 24a up and down, i.e., parallel to the Z axis which is vertical to the plane F of symmetry. Once the optimum position of the upper conical pole piece is obtained, bolt 35d and the other bolts 35a, 35b, 35c and 35e are also turned to press the upper conical pole piece and, hence, the upper collars onto the distant wedge-shaped pieces. This operation adjusts the dispersing magnetic field to be changed exactly in proportion to r in the plane of symmetry F in which the path of the central ion beam lies. Although the gap between the parallel pole pieces is simultaneously changed, the curvature radius of the ion beam in the focusing magnetic field is maintained equal to that of the ion beam in the dispersing magnetic field at all times, since the upper parallel pole face 36a which is intercrossing with the upper conical pole face 25a in a point directly above the path of the central ion beam is moved up and down, i.e., parallel to the vertical axis Z together with the upper conical pole face. Due to the common energizing magnet 3, the strength of the focusing magnetic field along the ion beam path is kept equal to that of the dispersing magnetic field along the ion beam path at all times, and the adverse effect of the fringing magnetic fields on the ion beam path is reduced to negligible proportions.

Thus, in the mass spectrograph with large mass dispersion constructed according to the present invention, the dispersing magnetic field strength along the ion beam path is kept equal to the dispersing magnetic field strength along the ion beam path by generating the two magnetic fields with a single energizing magnet 3 exactly by an excitation source 37. The time required to control the strength of the focusing magnetic field so that it varies in proportion to the strength of the dispersing magnetic field is reduced to a great extent.

Rapid scanning of the mass spectrum of a sample which requires the strengths of the two magnetic fields to be varied and at the same time the intensity ratio between the two magnetic fields to be kept constant at all times, has been made possible by connecting a scanner 38 to the excitation source 37.

While I have shown and described a preferred em-- bodiment of my invention, it may otherwise be embodied within the scope of the appended claims.

Iclaim:

1. In a rapid scanning double focusing mass spectrograph having an ion source, two focusing means and a dispersing means wherein an ion beam passes through a first focusing means to a dispersing means and then through a second focusing means prior to impingement upon a detector, the improvement comprising:

A. a means for producing a dispersing field and a second focusing field in which the curvature of radii are equal; said dispersing field being an r magnetic field and said second focusing field being an homogeneous magnetic field; and,

B. a means comprising a common electromagnetic core for exciting both fields simultaneously over a range of intensities such that the intensity ratio between both fields is maintained constant.

2; The improvement set forth in claim 1 wherein themeans for producing the dispersing. field, r magnetic field, comprise upper and lower confronting conical pole pieces symmetrically arranged about the ion beam orbit plane, said pole pieces having projection linesthat intersect on the orbit plane and vertical axis of the r and homogeneous magnetic fields, the centers of said po'le pieces lying above and below the central ion beam.

3. The improvement set forth in claim 2 wherein said means for producing the second focusing field,

homogeneous magnetic field, comprise upper and lower parallel pole pieces, the centers of said pole pieces lying above and below the central ion beam.

4. The improvement set forth in claim 3 wherein the upper pole pieces are joined together and the lower pole pieces are joined together.

5. The improvement set forth in claim 3 wherein the gap between the upper and lower pole pieces is adapted to vertical adjustment by adjusting means 6. The improvement set forth inclaim 5 wherein said adjusting means comprise upper. and lower collars mounted on the upper and lower conical pole pieces, nonmagnetic wedge-shaped elementsslidably mounted between. said collars, and means for moving said wedge-shaped elements inwardly and outwardly from 

1. In a rapid scanning double focusing mass spectrograph having an ion source, two focusing means and a dispersing means wherein an ion beam passes through a first focusing means to a dispersing means and then through a second focusing means prior to impingement upon a detector, the improvement comprising: A. a means for producing a dispersing field and a second focusing field in which the curvature of radii are equal; said dispersing field being an r 1 magnetic field and said second focusing field being an homogeneous magnetic field; and, B. a means comprising a common electromagnetic core for exciting both fields simultaneously over a range of intensities such that the intensity ratio between both fields is maintained constant.
 1. In a rapid scanning double focusing mass spectrograph having an ion source, two focusing means and a dispersing means wherein an ion beam passes through a first focusing means to a dispersing means and then through a second focusing means prior to impingement upon a detector, the improvement comprising: A. a means for producing a dispersing field and a second focusing field in which the curvature of radii are equal; said dispersing field being an r 1 magnetic field and said second focusing field being an homogeneous magnetic field; and, B. a means comprising a common electromagnetic core for exciting both fields simultaneously over a range of intensities such that the intensity ratio between both fields is maintained constant.
 2. The improvement set forth in claim 1 wherein the means for producing the dispersing field, r 1 magnetic field, comprise upper and lower confronting conical pole pieces symmetrically arranged about the ion beam orbit plane, said pole pieces having projection lines that intersect on the orbit plane and vertical axis of the r 1 and homogeneous magnetic fields, the centers of said pole pieces lying above and below the central ion beam.
 3. The improvement set forth in claim 2 wherein said means for producing the second focusing field, homogeneous magnetic field, comprise upper and lower parallel pole pieces, the centers of said pole pieces lying above and below the central ion beam.
 4. The improvement set forth in claim 3 wherein the upper pole pieces are joined together and the lower pole pieces are joined together.
 5. The improvement set forth in claim 3 wherein the gap between the upper and lower pole pieces is adapted to vertical adjustment by adjusting means. 